1. Field of the Invention
The present invention relates to a zoom lens and an image pickup apparatus equipped with the zoom lens, and more particularly, to a zoom lens suitable for a photographic optical system such as a still camera, a video camera, a digital still camera, a TV camera, and a monitoring camera.
2. Description of the Related Art
As an image pickup apparatus is miniaturized with a high functionality, there is a demand for a photographic lens used in the image pickup apparatus that is compact with a short lens length in entirety and provides a wide angle of view, a high zoom ratio, and a high resolution sufficient to appropriately correct chromatic aberration.
There is known a four-unit zoom lens including a first lens unit of a positive refractive power, a second lens unit of a negative refractive power, a third lens unit of a positive refractive power, and a fourth lens unit of a positive refractive power in order from an object side to an image side. In addition, there is also known a rear focus type four-unit zoom lens in which zooming is performed by moving the second lens unit, and the fourth lens unit corrects an image plane variation caused by zooming and performs focusing.
In general, the rear focus type zoom lens facilitates miniaturization of the entire zoom lens since the first lens unit has a smaller effective diameter in comparison with a zoom lens that performs focusing by moving the first lens unit. In addition, it facilitates near-distance imaging, particularly, when the imaging is performed at an extremely close range. Furthermore, since a lens unit having a small size and a light weight is moved, the lens unit can be moved with a small driving force, and focusing can be performed rapidly.
For example, U.S. Patent Application Publication No. 2010/0302651 and Japanese Patent Application Laid-open No. 2011-28144 discuss a miniaturized zoom lens including a first lens unit having a single negative lens and three positive lenses.
In order to obtain a predetermined zoom ratio and miniaturize the entire zoom lens, it is effective to increase the refractive power of each lens unit of the zoom lens and reduce the number of lenses. However, if the refractive power of each lens surface increase, the lens thickness increases. Accordingly, an effect of miniaturization of the lens system becomes insufficient, and various types of aberration are generated so that correction of such aberration is difficult.
In the positive lead type zoom lens, it is important to appropriately set each element of the zoom lens in order to miniaturize the entire zoom lens and obtain a high zoom ratio and high optical performance. For example, it is important to appropriately set the number of lens units, the refractive power of each lens unit, movement loci caused by zooming of each lens unit, zooming contributions of each lens unit, and the like.
In particular, in the positive lead type four-unit zoom lens, it is important to appropriately set each element of the first and second lens units. For example, it is important to appropriately set configurations such as materials of each lens of the first and second lens units, a refractive power (inverse of focal length) of the second lens unit for zooming, or a movement amount for zooming.
If such configurations are not appropriate, the size of the entire zoom lens increases to obtain a high zoom ratio. In addition, variations of various types of aberration caused by zooming, particularly, a variation of chromatic aberration increases, so that it is difficult to obtain excellent optical performance across the entire zoom range and the entire image plane.